Compressors that operate with pistons and cylinders use piston rods between a crankshaft or its equivalent and the piston in order to move the piston axially in reciprocating motion within the cylinder. One equivalent to a crankshaft is a swash plate which also converts the rotary motion of a power source to reciprocating motion of a piston within a cylinder. The crankshaft or swash plate is located in a crankcase and it is usually provided with a liquid oil lubrication system to lubricate the bearings between the crankshaft or the swash plate and the piston rods as well as other parts that move in a manner to create friction. In some compressors the lubricant is pumped to the points where lubrication is needed, and in other compressors lubrication splashes within the crankcase due to the movement of the crankshaft or swash plate. In either case the lubricant is subjected to high energy and it is subdivided into tiny droplets which eventually plate out on the interior surfaces of the crankcase and rejoin the main body of lubricant. Lubricant finds its way into the cylinders of the compressors and small amounts of it saturate the piston rings and eventually finds its way into the compressed gas stream.
Some compressed gases cannot tolerate oil contamination. For example, air that is compressed for breathing such as is used in SCUBA tanks or air that circulates in a confined space such as in a submarine can tolerate no oil contamination. Gases used for medical purposes or gases that react with oil must also be free of oil.
To keep crankcase oil from the cylinder of a compressor and, therefore, from the stream of compressed gas, it has been attempted to isolate the piston and cylinder in a chamber separated from the crankcase by a wall and to pass the piston rod or piston rod assembly through wipers surrounding the hole in the wall. A very successful means for accomplishing this objective is to position the piston rod within a sleeve that is sealed against the bottom of the piston so that the sleeve reciprocates axially through the wiper even though the piston rod sweeps out a shape such as a fan or a cone. This device is the subject of my co-pending patent application entitled OIL-SHIELDED COOMPRESSOR, Ser. No. 545,036, filed Jun. 25, 1990.
A chamber so isolated from the crankcase is subjected to pulsing pressures due to the action of the piston. When the piston moves through its compression stroke it increases the volume of the chamber and thereby reduces the pressure in the chamber, while when the piston is in its intake stroke it will reduce the volume of the chamber and thereby increase pressure in the chamber. There is always a small flow of gas into the chamber due to blow-by past the piston ring and in compressor stages operating at very high pressure this blow-by may be more significant than in low pressure stages. The condition of the piston rings will also cause the amount of blow-by to vary. In order to avoid an unacceptable build-up of pressure the chamber is vented.
Although the chamber beneath the piston has an average super-atmospheric pressure, the pressure pulsations produce some periods of sub-atmospheric pressure in the chamber; and during those sub-atmospheric periods, oil tends to be blown through the wipers from the crankcase into the chamber. As the wiper elements become contaminated oil is drawn into the chamber, and it eventually finds its way into the compressed gas stream by being carried up the walls of the cylinder by piston rings that become contaminated with oil.